1. Field of the Invention
The present invention relates to a method of carrying out operations in underground/subsea oil/gas wells, preferably by the utilisation of coiled tubing to carry the work tool. More specifically, this method is meant to be used for advancing a rotating downhole tool in an underwater well, wherein said tool is brought to rotate by means of a downhole motor carried by the coiled tubing. Thereby, the method is of the kind specified in the introduction of claim 1.
2. Description of the Related Art
Also, the invention relates to an apparatus of the kind, which may be employed to implement or support the effect of the method according to the invention, and which comprises a motorized downhole tool, which is arranged to be connected to a pipe string/rod string, preferably coiled tubing, and to receive the torque for the rotation of the tool from the motor. The apparatus according to the invention is thereby of the kind appearing in further detail from the introductory part of the following first independent claim to the apparatus.
Also, the invention comprises a particular application of the method/apparatus.
When the exploitation of a sea-based oil/gas field is considered no longer financially profitable, and the underwater wells are about to be shut down and abandoned, the wells are to be plugged in a reliable manner.
To ensure proper plugging of each of the underwater wells by grouting, the inner casing (run last) must be withdrawn, so that cement mixture can be filled all the way out to the wall of the well. It is not sufficient to fill cement mixture into the inner casing, because formation fluid penetrating into the annulus, could penetrate further up and out of the well if the cement mixture, which has surrounded the casings already from the cementing thereof, is not tight.
To withdraw the (inner) casing, break it up and transport it to shore is very laborious. Therefore, the oil companies are interested to find a solution, whereby the casing will remain in situ, while at the same time, the well is plugged in accordance with regulations.
This can be achieved by running a cutting tool into the well, cutting away the inner casing in an area below the other casings. A rotating cutting tool is lowered into the casing to the desired depth, where the pivotal blades of the tool are folded out gradually, cutting the casing. Then the tool is displaced in the well while it is rotating and milling and drilling out the casing from the end at the cutting point. When about 15 meters of the casing wall has been drilled out and milled away, the operation is completed, and the equipment can be pulled up. Then, when cement mixture is filled into the inner casing, the cement mixture can penetrate all the way out to the formation in the area from which the casing has been milled away.
Several solutions for milling/drilling tools have been suggested (milling tools, grinding or chipping tools, normally arranged to be mounted in the place of the drill bit).
Since, in general, there are no drill rigs on the platforms normally employed for the implementation of the operations relevant in connection with plugging of underwater wells, which are to be abandoned, it is desirable to be able to use coiled tubing to enter the well with tools. The alternative is to mount a drill rig on the platform, but that is both expensive and time-consuming.
However, coiled tubing will not be able to absorb sufficient torque from the cutting/milling/drilling tool like an ordinary drill string could have done, and thus it is imperative to have extra torque-absorbing equipment mounted in association with the coiled tubing.
In the technical field of the present invention the insufficient capacity of coiled tubing to absorb torques is considered a qualified problem in connection with motorised rotating downhole tools.
A previously known suggestion, which oil companies have found interesting, involves anchoring a hydraulic piston-and-cylinder, with a piston travel of a couple of meters, at the end of the coiled tubing, and securing an assembly comprising tools with a motor arranged thereto, to the end of the piston rod of the piston-and-cylinder.
In the execution of said downhole operation by means of the rotating motorised tool, a hydraulically expanding clamping ring (or other expanding clamping device) provides for fixing the piston-and-cylinder in the casing and absorbing the torque from the driven rotating tool, while the piston-and-cylinder causes advancing of the tool.
When the piston-and-cylinder has advanced the tool a distance corresponding to a length of stroke, the expanding clamping ring is released, and the apparatus (downhole tool+driving motor) is moved forward a distance corresponding approximately to a length of stroke in the direction of advancing. The clamp ring is tightened again, and the tool is displaced to the milled end of the casing, and the process is repeated.
However, an ordinary hydraulic piston-and-cylinder, in which the piston and piston rod have circular cross-sections, cannot absorb any torque. Therefore, also in this known device extra measures are necessary to handle the torques, such as formation of longitudinal grooves in the piston rod and the slip at the end gable of the cylinder, or so-called splines (grooves, flutes etc.), a particular guide rail or other means can be used. This complicates the equipment and it will all be very expensive.
In accordance with the present invention it has been established, among other things, that apart from its inability to absorb torques, coiled tubing exhibits considerable strength properties and is more than strong enough to endure the advancing force proper.
Thereby the general object of the invention has been to reach and prescribe a method of the kind specified in the introductory part of claim 1, whereby, based on simple operational steps, the drawbacks described in the preceding are remedied, and whereby also in other respects, a technique advantageous in terms of work and time and also economy, is obtained.